1. Field of the Invention
The present invention relates to a power supply and, more specifically, to a power supply system that combines power output from two or more powers supplies.
2. Description of the Related Art
Many electronic devices tend to require much more sophisticated power supplies for supplying power. For example, many electronics may require high frequency, high overall efficiency, fewer components, and/or low ripple in the power supplied by the power supplies.
More specifically, there is often a need for a power supply circuit that is capable of delivering power with high frequency components (fast changing voltage and current), at high overall power conversion efficiency. For example, an RF (Radio Frequency) PA (power amplifier) can be fed by an efficient power supply at a reduced voltage, allowing the PA to operate more efficiently (i.e., with lower power consumption). In these RF power amplifiers, the power supply must be capable of changing the output voltage very quickly to accommodate rapid changes in the output power of the PA, requiring the power supply to deliver high frequency components of power. At the same time, a high overall efficiency is desired in the power supply to achieve the desired lower power consumption. A typical switched-mode power supply (SMPS) circuit achieves high efficiency, but cannot deliver sufficiently high frequency components of the power, because the low switching frequencies commonly used in these types of regulators (a limitation largely imposed by the magnetics) limits the regulator's bandwidth. Linear regulators, on the other hand, may be designed to deliver high frequency components, but the power conversion efficiency of such a linear regulator is poor. Thus neither a common SMPS nor a linear regulator can meet this need.
Another example of the need for a power supply that is both efficient and can deliver a fast changing voltage and current is one which supplies a digital circuit, which may include a microprocessor. The digital circuit may operate more efficiently if fed by a power supply that adjusts its voltage dynamically to match the predicted processing needs. Typically, the voltage is adjusted upwards when the digital circuit is operating at high speeds, and downward when operating at lower speeds. While conventional power supplies can typically change their voltage within 50 μs, this delay may prevent the digital circuitry from operating at peak efficiency, and a power supply which adjusts its voltage more quickly to allow for a more frequent change in clocking speeds of the digital circuitry is desirable.
There is also a need to reduce the number of components used in switching power supply circuits, such as output capacitors and inductors, and the associated costs of using such capacitors and inductors. The output capacitors often used are a large value, a low-ESR (Equivalent Series Resistance) type, requiring the use of large electrolytic capacitors with exotic electrolytes. These output capacitors are required to reduce the ripple voltage caused by repetitive currents from the switching action of the regulator appearing at the output of the power supply. The quantity and quality of these capacitors add greatly to the cost of the power supply, and the volume added to by these capacitors may be unattractive for use in portable electronic devices. Additionally, the low-ESR capacitors may also be undesirable, as the electrolytes can be flammable and create a fire hazard. In addition, some high-current switching regulator circuits use several inductors, with a controller which phases the switching of the inductors to reduce output ripple. The use of multiple inductors undesirably adds to the cost of the power supply.
Further, a low voltage ripple is desirable in switching power supplies. For example, modern microprocessors are increasingly operated at low voltages due to increased chip density and lower voltage breakdown in advanced CMOS (Complementary Metal Oxide Semiconductor) technology. At these low voltages, the power supply ripple may be a substantial portion of the supply voltage. High ripple may undesirably require the power supply output voltage to be raised above the optimal level in order to ensure that the microprocessor is supplied with the minimal voltage required during periods when the ripple voltage drives the voltage excursions to a minimum. As an additional example, a RF PA requires its power supply to exhibit low ripple at its output. Ripple typically occurs synchronously to the switching frequency of the switching regulator and can feed through to the output of the PA, causing unwanted distortion in the RF output signal.
There have been some efforts to improve the conventional switching regulator circuits. For example, one could use both a switching regulator and a linear regulator and have a simple summing node to combine outputs from the linear regulator and the switching regulator to form the output of the power supply, with the intention that the linear regulator provides the high frequency, and the switching regulator provides the low frequency and DC components of the current to the load. These circuits, however, place a lot of burden on the linear regulator, as it requires the linear regulator to supply a large amount of excess current to modulate the voltage in the large capacitors needed by the switching regulator. Alternatively, a switching regulator and linear regulator may be placed in series, with the switching regulator's output feeding the linear regulator's input. In this arrangement, the linear regulator may be capable of delivering high frequency components of the power, while the switching regulator may deliver power efficiently to the linear regulator. However, this series arrangement forces all the power delivered to the load to pass through the linear regulator, causing dissipation in the linear regulator and substantially reducing the overall efficiency of the power supply.
Therefore, there remains a need for a power supply system that has high overall efficiency, high bandwidth, and low voltage ripple, and one that uses a reduced number of components.